This invention relates to an electric field detector in an electrically heatable transparency and in particular to a detector usable with a heatable windshield that senses a change in the voltage in the electrically heatable coating of the windshield which would be associated with a break in the leads to the bus bars and interrupt electrical power to the windshield to prevent arcing and additional damage to the heatable windshield.
2a. Technical Considerations
It has been known to pass electric current through a bus bar and transparent conductive coating to another bus bar on the surface of a transparency to raise its temperature. Automotive transparencies, and in particular heatable windshields, include an electrically conductive coating on a major surface of the windshield, interconnected by a pair of opposing bus bars. Typical heatable windshield configurations include either single or dual power lead to the bus bars. In the former configuration, the opposing bus bars are each connected to a single power lead. In the latter configuration, power is provided to at least one of the bus bars by dual leads, each attached to opposite ends of the bus bar. In this fashion, if one of the leads fails, the other can still supply power to the bus bar and maintain the windshield's operation.
One mode of failure of heatable transparencies is localized overheating of the bus bars and/or leads which results from arcing when current moves through a defective power lead or bus bar. In the instance where the transparency is a laminated windshield, the overheating caused by arcing can damage the plastic interlayer, or in severe cases, damage the laminated transparency thus adversely affecting its occupant retention capability.
It has been suggested to detect power discontinuities in the bus bar leads by using a sensor lead that either extends along a bus bar power lead and is connected to the bus bar system at the juncture between the power lead and bus bar or at some other location along the bus bar. The current passing through the power lead to the bus bar is monitored via the sensor line for a predetermined voltage drop which would indicate that the lead adjacent the sensor line is damaged. However, such a sensor arrangement would not detect a discontinuity in the lead to the other bus bar since the sensor line only monitors the current to one of the bus bars. In addition, this sensor configuration used in combination with a dual feed bus bar configuration would effectively negate one of the advantages of using a dual feed configuration, namely a break in the sensor line adjacent one of the power leads to the dual feed bus bar would terminate power to the windshield even though the other lead to the bus bar allows the windshield to continue operation.
It would be advantageous to have the capability of detecting a defect in either bus bar power lead in the electrical heating system of a heatable windshield of the type which may result in overheating and interrupting current flow to the bus bars before the windshield is permanently damaged.
2B. Patent of Interest
U.S. Pat. No. 3l876,862 to Newman et al. teaches a circuit breaker for an electrically energized glass panel. An electroconductive coating interconnects pairs of opposing bus bars and a frangible strip extends around the periphery of the bus bars and coating. When the frangible strip breaks, electrical power to the panel is interrupted.
U.S. Pat. No. 3,892,947 to Strengholt teaches an electrically heated panel with anti-shock conductive strips. An electroconductive strip which functions as a bus bar extends around the periphery of the panel before contacting any portion of an electrically conductive coating on the panel surface. A single break or fracture in the strip will prevent any current from flowing to the coating.
U.S. Pat. No. 4,057,671 to Shoop, U.S. Pat. No. 4,323,726 to Criss et al., and U.S. Pat. Nos. 4,543,466 and 4,668,270 to Ramus teach a heated laminated window with an electroconductive coating or a wire grid extending between a pair of spaced bus bars. Current passes from one bus bar, through the coating or wire grid, to the opposite bus bar to heat the window.
U.S. Pat. No. 4,565,919 to Bitter, et al, teaches a crack detector circuit that detects a crack in the electroconductive coating in a heatable windshield. The circuit monitors the resistance of the coating by comparing the voltage applied to the electroconductive coating through one bus bar to the output voltage of the coating through the opposing bus bar. A control switch terminates power to the coating when this proportion assumes an unacceptable value indicating that the coating has cracked.
U.S. Pat. No. 4,808,799 to Schave and U.S. Pat. No. 4,829,163 to Rausch et al. teach a crack detector for a heatable windshield having an electroconductive coating which electrically interconnects a pair of opposing bus bars. A crack detector extends along the lead to the upper bus bar and is electrically interconnected to either the upper bus or the lead at the junction between the upper bus bar and the lead. The voltage in the lead or upper bus bar is monitored through the crack detector which terminates power to the windshield if the monitored voltage of the lead or upper bus bar differs from a reference voltage by a predetermined amount.